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Synthesis, characterization and photophysical properties of first-row transition metal complexes
No full textAbstract
According to Horizon 2020, Europe's photonics industry is strong (e.g. in laser-based manufacturing, medical photonics, sensing, lighting) and has the possibility to exploit new emerging market opportunities. Advanced lighting technology includes the use of Solid State Lighting (SSL) based on organic light-emitting diodes (OLEDs). The advantages involve higher quality lighting in terms of energy efficiency, quality (robustness, longer lifetime, colour tunability), and therefore cost reduction and energy saving. Lighting represents around 19% of electricity consumption worldwide and the replacement of old lighting technologies such as incandescent bulbs with SSL systems will allow to save up to 70% of energy.
With all of this in mind, it is evident that the development of light technology based on SSL is of paramount importance. Nowadays most of the luminescent complexes applied in SSL are based on lanthanides and d-block metals of the second and third transition series. Platinum(II) and iridium(III) are commonly used as emitters for OLEDs due to their long-lived triplet states. However, recent studies have demonstrated that it is possible to obtain highly luminescent complexes based on first-row transition elements, such as chromium(III), iron(III), manganese(II), copper(I) and zinc(II). These elements have the advantage to be less expensive and toxic, as well as more abundant.
The main focus of the following PhD thesis is the synthesis and characterization of luminescent manganese(II), copper(I) and zinc(II) complexes. As concerns the first one, the metal-centred emission related to the 4T1(4G) → 6A1(6S) transition is strongly dependent upon the coordination sphere: tetrahedral complexes are normally green emitters, while octahedral species emit in the red range. The presence of rigid structures and light harvesting fragments in the ligand skeleton allows to enhance the luminescent features as well as the UV-light absorption. Several [O=P]-donor ligands were considered for the preparation of both tetrahedral and octahedral derivatives, sometimes affording dual emissions in the corresponding manganese(II) complexes due to concurrent metal- and ligand-centred transitions.
As regards copper(I) complexes, derivatives containing polydentate N-donors based on indazole and benzotriazole were synthesized and their emission properties were attributed to metal-to-ligand charge transfer mechanisms involving triplet emitting states. Small changes in the ligand skeleton determined appreciable variations in the photophysical properties, as testified by the complexes obtained using 2,1,3-benzothiadiazole as N-donor.
The [O=P]-donors previously used for manganese(II) revealed to be suitable ligands also for the preparation of luminescent zinc(II) halide complexes thanks to the enhancement of ligands fluorescence due to coordination. In selected cases intersystem crossing was observed, causing intense green phosphorescence or dual emission.According to Horizon 2020, Europe's photonics industry is strong (e.g. in laser-based manufacturing, medical photonics, sensing, lighting) and has the possibility to exploit new emerging market opportunities. Advanced lighting technology includes the use of Solid State Lighting (SSL) based on organic light-emitting diodes (OLEDs). The advantages involve higher quality lighting in terms of energy efficiency, quality (robustness, longer lifetime, colour tunability), and therefore cost reduction and energy saving. Lighting represents around 19% of electricity consumption worldwide and the replacement of old lighting technologies such as incandescent bulbs with SSL systems will allow to save up to 70% of energy.
With all of this in mind, it is evident that the development of light technology based on SSL is of paramount importance. Nowadays most of the luminescent complexes applied in SSL are based on lanthanides and d-block metals of the second and third transition series. Platinum(II) and iridium(III) are commonly used as emitters for OLEDs due to their long-lived triplet states. However, recent studies have demonstrated that it is possible to obtain highly luminescent complexes based on first-row transition elements, such as chromium(III), iron(III), manganese(II), copper(I) and zinc(II). These elements have the advantage to be less expensive and toxic, as well as more abundant.
The main focus of the following PhD thesis is the synthesis and characterization of luminescent manganese(II), copper(I) and zinc(II) complexes. As concerns the first one, the metal-centred emission related to the 4T1(4G) → 6A1(6S) transition is strongly dependent upon the coordination sphere: tetrahedral complexes are normally green emitters, while octahedral species emit in the red range. The presence of rigid structures and light harvesting fragments in the ligand skeleton allows to enhance the luminescent features as well as the UV-light absorption. Several [O=P]-donor ligands were considered for the preparation of both tetrahedral and octahedral derivatives, sometimes affording dual emissions in the corresponding manganese(II) complexes due to concurrent metal- and ligand-centred transitions.
As regards copper(I) complexes, derivatives containing polydentate N-donors based on indazole and benzotriazole were synthesized and their emission properties were attributed to metal-to-ligand charge transfer mechanisms involving triplet emitting states. Small changes in the ligand skeleton determined appreciable variations in the photophysical properties, as testified by the complexes obtained using 2,1,3-benzothiadiazole as N-donor.
The [O=P]-donors previously used for manganese(II) revealed to be suitable ligands also for the preparation of luminescent zinc(II) halide complexes thanks to the enhancement of ligands fluorescence due to coordination. In selected cases intersystem crossing was observed, causing intense green phosphorescence or dual emission
Visible-emitting Cu(I) complexes with N-functionalized benzotriazole-based ligands
Full text linkLuminescent mono- and dinuclear cationic heteroleptic Cu(I) complexes [Cu(NN'
)(P)2]+, [Cu(NN')(PP)]+ or [Cu2(NN')2(m-PP)2]2+ containing bidentate N-donor ligands (NN') with benzotriazole, pyridine, pyrimidine or substituted triazine moieties in combination with mono- (P) and bidentate (PP) phosphines were synthesized and characterized. Eight single-crystal X-ray diffraction structures were obtained and showed marked distortions from the ideal tetrahedral geometry around Cu(I). Cyclic voltammetry on selected complexes showed reduction processes around 2 V vs. ferrocene/ferrocenium and irreversible oxidation close to 1 V. The long-wavelength absorptions were observed in the range of 350 to 450 nm and attributed to MLCT transitions. Upon excitation with near-UV and violet light, the complexes exhibited emissions from bright yellow (max 538 nm) to red (max 637 nm). Emission maxima, luminescence lifetimes and photoluminescence quantum yields that reach up to 0.92 on powder samples resulted in strong dependence on the choice of the coordinated ligands, the acceptor character of the NN' ligands in
particular. DFT calculations confirmed the electrochemical and photophysical outcomes and strongly suggested that the emission has a metal-to-ligand charge transfer (MLCT) nature, with intersystem crossing affording triplet emitting states
Preparation, reactivity and photoluminescence of copper(I) borohydride complexes with bis[(2-diphenylphosphino)phenyl] ether as chelating ligand
Full text linkBis[(2-diphenylphosphino)phenyl] ether (DPEphos) was used as chelating ligand to prepare the corresponding borohydride complex [Cu(κ2–BH4)(DPEphos)], whose structure was ascertained spectroscopically and by means of X-ray diffraction. The spectroscopic assignments related to the coordinated borohydride were confirmed by preparing the isotopologue [Cu(κ2–BD4)(DPEphos)]. Reaction of [Cu(κ2–BH4)(DPEphos)] with triflic acid afforded the dimer [Cu2(μ–BH4)(DPEphos)2][OTf] (OTf = triflate). The borohydride complexes exhibited appreciable blue emission upon excitation with UV light at room temperature. [Cu(κ2–BH4)(DPEphos)] revealed to be a suitable precursor for the preparation of luminescent heteroleptic copper(I) complexes having general formula [Cu(N^N)(DPEphos)]+ (N^N = 1,10-phenantroline, 2,9-dimethyl-1,10-phenantroline, 2,2’-bypiridine, 4,4’-dimethyl-2,2’bipyridine)
Luminescent heteroleptic copper(I) complexes with polydentate benzotriazolyl-based ligands
Full text linkBis(benzotriazol-1-yl)phenylmethane CHPh(btz)2 and tris(benzotriazol-1-yl)methane CH(btz)3 were used as N-donor ligands to prepare luminescent heteroleptic copper(I) complexes. [Cu{CHPh(btz)2}(PPh3)2][BF4] and [Cu{CHPh(btz)2}(DPEphos)][BF4] (DPEphos = bis[(2-diphenylphosphino)phenyl] ether) were obtained from the corresponding borohydride complexes [Cu(BH4)(PPh3)2] and [Cu(BH4)(DPEphos)] and tetrafluoroboric acid. [Cu{CH(btz)3}(PPh3)][BF4] and [Cu{CH(btz)3}(PiPr3)][BF4] were prepared from the acetonitrile complex [Cu(NCCH3)4][BF4]. The complexes exhibited bright yellow or orange emissions upon excitation with near-UV and violet light. The photoluminescent properties were attributed to metal-to-ligand charge transfer transitions on the basis of experimental data and DFT calculations
Dual-emitting Mn(II) and Zn(II) halide complexes with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide as ligand
Full text linkThe H-phosphinate 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) acts as O-donor ligand towards manganese(II) and zinc(II) halides, with the formation of tetrahedral complexes having general formula [MX2(DOPO)2] (M = Mn, X = Cl, Br, I; M = Zn, X = Br). The structure of [ZnBr2(DOPO)2] was ascertained by single-crystal X-ray diffraction. All the compounds showed appreciable photoluminescence at the solid state, with an emission band in the red region of the spectrum attributed to radiative decay from triplet states of the coordinated DOPO ligands. Such an emission is superimposed to the 4T1(4G)→6A1(6S) transition around 530–540 nm in the case of the Mn(II) derivatives and to a fluorescent band centred at 380 nm for [ZnBr2(DOPO)2]. Excitation wavelength-dependent emission was observed for [MnCl2(DOPO)2] and [MnBr2(DOPO)2]. The absorption and emission features of the coordinated DOPO ligands were rationalized by means of TD-DFT calculations
Impact of COVID-19 in Children with Chronic Lung Diseases
Full text linkBackground: since December 2019, the world has become victim of the coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). The aim of our narrative review is to analyze the impact of COVID-19 in children suffering from chronic lung disease (CLD). Methods: we searched the MEDLINE/Pubmed database using the terms “SARS-CoV-2” or “COVID-19” or “Coronavirus Diseases 2019”; AND “chronic lung diseases” or “chronic respiratory diseases” or “asthma” or “cystic fibrosis” or “primary ciliary dyskinesia” or “bronchopulmonary dysplasia”; and limiting the search to the age range 0–18 years. Results and Conclusions: although COVID-19 rarely presents with a severe course in children, CLD may represent a risk factor; especially when already severe or poorly controlled before SARS-CoV-2 infection. On the other hand, typical features of children with CLD (e.g., the accurate adoption of prevention measures, and, in asthmatic patients, the regular use of inhaled corticosteroids and T2 inflammation) might have a role in preventing SARS-CoV-2 infection. Moreover, from a psychological standpoint, the restrictions associated with the pandemic had a profound impact on children and adolescents with CLD
Single-Crystal X-ray Structure Determination of Tris(pyrazol-1-yl)methane Triphenylphosphine Copper(I) Tetrafluoroborate, Hirshfeld Surface Analysis and DFT Calculations
Full text linkThe tetrafluoroborate salt of the cationic Cu(I) complex [Cu(CHpz3)(PPh3)]+, where CHpz3 is the tridentate N-donor ligand tris(pyrazol-1-yl)methane and PPh3 is triphenylphosphine, was synthesized through a displacement reaction on the acetonitrile complex [Cu(NCCH3)4][BF4]. The compound crystallizes in the monoclinic P21/c space group. The single-crystal X-ray diffraction revealed that the copper(I) centre is tetracoordinated, with a disposition of the donor atoms surrounding the metal centre quite far from the ideal tetrahedral geometry, as confirmed by continuous shape measures and by the τ4 parameter. The intermolecular interactions at the solid state were investigated through the Hirshfeld surface analysis, which highlighted the presence of several non-classical hydrogen bonds involving the tetrafluoroborate anion. The electronic structure of the crystal was modelled using plane-wave DFT methods. The computed band gap is around 2.8 eV and separates a metal-centred valence band from a ligand-centred conduction band. NMR spectroscopy indicated the fluxional behaviour of the complex in CDCl3 solution. The geometry of the compound in the presence of chloroform as implicit solvent was simulated by means of DFT calculations, together with possible mechanisms related to the fluxionality. The reversible dissociation of one of the pyrazole rings from the Cu(I) coordination sphere resulted in an accessible process
Luminescent Behavior of Zn(II) and Mn(II) Halide Derivatives of 4-Phenyldinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepine 4-Oxide and Single-Crystal X-ray Structure Determination of the Ligand
Full text linkThe two enantiomers of chiral phosphonate 4-phenyldinaphtho[2,1-d:1′,2′-f][1,3,2]dioxaphosphepine 4-oxide, O=PPh(BINOL), were synthesized from the proper 1,1′-bi-2-naphtol (BINOL) enantiomer and characterized. The structure of the (S)-enantiomer was elucidated by means of single-crystal X-ray diffraction. The reaction with anhydrous ZnBr2 afforded complexes having the general formula [ZnBr2{O=PPh(BINOL)}2] that showed intense fluorescence centered in the near-UV region rationalized on the basis of TD-DFT calculations. The corresponding Mn(II) complexes with the general formula [MnX2{O=PPh(BINOL)}2] (X = Cl, Br) exhibited dual emission upon excitation with UV light, with the relative intensity of the bands dependent upon the choice of the halide. The highest energy transition is comparable with that of the Zn(II) complex, while the lowest energy emission falls in the red region of the spectrum and is characterized by lifetimes in the hundreds of
microseconds range. Although the emission at lower energy can also be achieved by direct excitation of the metal center, the luminescence decay curves suggest that the band in the red range is possibly derived from BINOL-centered excited states populated by intersystem crossing
Isocyanide Cu(I) complexes with unexpected μ2‐bridging pseudohalides: Synthesis, characterization and catalytic activity towards CuAAC
Full text linkThree neutral Cu(I) complexes bearing 2,6-dimethylphenyl isocyanide (CNXyl) and different triatomic pseudohalogens (SCN−, OCN− and N3−) as ligands were efficiently synthesized and characterized. The solid-state structures were unambiguously determined through single-crystal X-ray diffraction, revealing unexpected bridging coordination modes in the case of OCN− and N3−. All the complexes were tested for azide-alkyne cycloaddition (CuAAC), showing interesting catalytic activity towards the formation of 1,4-disubstituted-1,2,3-triazoles for the cyanato and the azido Cu(I) complexes. Both species afforded yields above 90% with 0.5 mol% of catalyst at 50°C for 24 h. Several alkynes and azides were tested using the more active azido Cu(I) complex, affording the corresponding triazoles in high yields. The azido Cu(I) complex also induced the intramolecular CuAAC in the presence of dimethyl acetylenedicarboxylate and benzyl bromide/phenylacetylene
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